Cause of grabbing brakes

[Reckless Rat]

Dave, whilst the drums may well be round, has the relationship between the drums, the brake shoes and the cams been verified? If you cover the lining material with chalk you can very quickly determine which parts of the brake shoe are actually contacting the inner surface of the drum as where it touches will be cleaned off. I suspect that the contact area isn't as great as it should be. The leading shoes may be the culprit as they have a self-servo action.

Just my two-penn'orth
Bruce